About the Optotrak system

Disclaimer: This page is by far no substitute for any of the documentation released by Northern Digital. It is intended as a mere introduction to the Optotrak system, in a similar manner how it would happen at a convention or a conference. Do not exclusively rely on this documentation. If you want to skip this, just go and look at the examples.

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What is the Optotrak system, and how does it work?

The Optotrak system is a camera-based motion capture system released by Northern Digital, and the Optotrak is a particular product line of motion capture systems they sell. It uses infrared light emitting diodes (IR LEDs) as the position markers. When an IR LED is on, it will be seen by a camera (or, as per the user guide, a 'position sensor'). Since a position sensor has three sensors in it which are placed slightly apart, the three images of the marker with the IR LED in it will be slightly different. The marker will be seen as a 'blob' in the cameras' image. It is possible to work with this extracted, 'raw' or 'centroid' data, but also the Optotrak system can use a few different algorithms to determine where that marker is, and return 3D coordinates.

Normally users wouldn't just want to use a single marker, but many markers in the same time. The Optotrak Certus system supports a maximum of 512 markers. If these markers were on all the time, it will be nearly impossible to separate which marker belongs to which centroid in the raw data, so they decided to use time division with the markers. With the use of a device they call a strober, it is possible to sequentially activate the markers for a short period of time, and hence quickly capture the position of a large number of markers. The position sensor and the strober are connected via the System Control Unit (SCU), and the SCU is connected to the computer where the system is configured and the data is saved to.

Sometimes a marker is not visible to the camera. It may well be that the marker is too dim because driving voltage and the duty cycle are too low, or it might be blocked from view to the position sensor. In either case, the position data is invalid, and this toolbox returnins a not-a-number (NaN) in Matlab. If you want to reduce the chance of a marker becoming invisible, you may want to relocate the marker to somewhere more convenient, and do not change the system's default setting of automatically adjusting the marker brightness. Alternatively you could add an other camera to the system.

Scalability and flexibility

One of the key features of the Optotrak system that it allows you to tailor it to your needs. You can connect up to eight cameras and use them simultaneously. There are pre-defined tools and rigid bodies available, which you can use for tracking. Note that even though you can connect 8 cameras (24 sensors), the API seems to have a limit of using up to 10 sensors at the same time.

In addition to the position sensors and the markers, it is also possible to connect an Optotrak Data Acquisition Unit (ODAU) to the system, which allows you to record analogue and digital signals along with the position data. If you have an ODAU connected, you can only connect up to six cameras at the same time.

How to connect the Optotrak system to the host computer

You can connect to the system in three ways:

• PCI card

You can get the PCI card, with a maximum theoretical bandwidth of 1064 Mbit/s, but the driver only works on 32-bit systems

• Ethernet link

You can use the Ethernet port on the SCU to get up to 100 Mbit/s bandwidth, but you'll need to configure the SCU's Ethernet interface manually via the RS-232 port.

• USB link

You can get an adaptor that connects to the link port. It uses an FTDI chip, but the drivers are modified by NDI. Do not use the FTDI driver. While the USB 2.0 standard provides 480 Mbit/s bandwidth, the FTDI chip can only handle a fraction of it.

A note on required bandwidth

I noticed that the FTDI's chip used in the USB adapter has a maximum effective bandwidth of about 2.4Mbit/s. The actual bandwidth required is depending on the number of markers, the sampling rate and the data type being transferred. It's in the range of a couple hundred kilobits per second. For exact figures, please read the manual.